The present invention relates to a stylus counterbalancing mechanism for a probe.
In measuring probes, in particular analogue measuring probes, in which the probe stylus is mounted on a spring suspension system within the probe itself, a problem arises when the probe stylus is changed for one of different weight, in that the change in weight can cause a deflection of the stylus mounting mechanism. The effect is a change in the zero position of the stylus requiring a re-calibration of the probe.
The more sensitive the probe is, the greater can be the effect of different weights of styli, to the extent that a heavy stylus can so deflect the stylus mounting, that all of the travel of the stylus which is required for making measurements can be used up.
A related problem occurs in touch trigger probes which have a mechanical rest position, in that changing the weight of the stylus changes the force required to move it from the rest position, which in turn will vary the pre-travel of the stylus, i.e. the amount of movement of the stylus tip before a signal relating to stylus movement is generated. These problems are well-known and various types of stylus counterbalancing mechanisms are known. These range from manual mechanisms for changing the compression of a spring which forms part of the stylus mounting, to fully automated mechanisms such as are described, for example, in U.S. Pat. No. 4,937,948.
In the above-mentioned patent specification there is described a stylus counterbalancing mechanism which comprises a pair of springs connected to a flange on a movable part of the stylus support. The measuring transducers within the probe are set to read zero when the attached stylus is out of contact with a workpiece. If the stylus is removed and a new one of different weight attached, the measuring transducers will read a positive or negative deflection. This change is used to generate a signal, which is used, in turn, to drive a motor in the appropriate sense to move the stylus support to bring the reading of the measuring transducers back to zero.
While the above-described mechanism provides for automatic adjustment of the stylus support for counterbalancing stylus weight, it adds the complication and expense of a motor and its control system within the probe. The motor also generates heat within the probe during its operation which is a disadvantage.